METHODS AND SYSTEMS FOR REDUCING NITROGEN OXIDES (NOx) IN A HEAT GENERATION UNIT USING SOUR WATER STRIPPER VAPOR

Abstract

Methods and systems for reducing NO x in a heat generation unit are provided. A method includes introducing an exhaust gas from a catalytic cracking unit to a combustion zone of a heat generation unit to produce a combusted exhaust gas, wherein the exhaust gas contains two or more of carbon monoxide, hydrogen cyanide, ammonia, and nitrogen oxide. The method further includes introducing a sour water stripper (SWS) vapor stream from a SWS unit to the heat generation unit at a location after the combustion zone and before a heat recovery zone of the heat generation unit. The method also includes allowing the SWS vapor stream to react with the combusted exhaust gas to produce a processed exhaust gas with decreased NO x content as compared to the NO x content when the exhaust gas is processed under similar conditions but without an interaction with the SWS vapor stream.

What is claimed is: 
     
         1 . A method for reducing NO x  in a heat generation unit, the method comprising:
 introducing an exhaust gas from a cracking unit to a combustion zone of the heat generation unit to produce a combusted exhaust gas, the exhaust gas containing two or more of carbon monoxide (CO), hydrogen cyanide (HCN), ammonia (NH 3 ), and NO x ;   introducing a sour water stripper (SWS) vapor stream from a SWS unit to the heat generation unit at a location after the combustion zone and before a heat recovery zone of the heat generation unit; and   allowing the SWS vapor stream to react with the combusted exhaust gas to produce a processed exhaust gas with a decreased NO x  content compared to a NO x  content when the exhaust gas is processed under similar conditions but without an interaction with the SWS vapor stream.   
     
     
         2 . The method of  claim 1 , wherein the SWS vapor stream is introduced in at least two temperature sections of the heat generation unit, the temperature sections being defined by a range of temperature of the combusted exhaust gas, and the at least two temperature sections being a first temperature section ranging from about 1600° F. to about 1800° F. and a second temperature section ranging from about 1800° F. to about 2200° F. 
     
     
         3 . The method of  claim 1 , wherein the SWS vapor stream is introduced in at least three temperature sections of the heat generation unit, the temperature sections being defined by a range of temperature of the combusted exhaust gas, and the at least three temperature sections being a first temperature section ranging from about 1600° F. to about 1800° F., a second temperature section ranging from about 1800° F. to about 1900° F., and a third temperature section ranging from about 1900° F. to about 1955° F. 
     
     
         4 . The method of  claim 1 , wherein the cracking unit is a fluid catalytic cracking unit. 
     
     
         5 . The method of  claim 1 , wherein the NO x  content in the processed exhaust gas is about thirty percent less than the NO x  content when the exhaust gas is processed under similar conditions but without the interaction with the SWS vapor stream. 
     
     
         6 . The method of  claim 1 , wherein the NO x  content in the processed exhaust gas is about fifty percent less than the NO x  content when the exhaust gas is processed under similar conditions but without the interaction with the SWS vapor stream. 
     
     
         7 . The method of  claim 1 , wherein the NO x  content in the processed exhaust gas is about seventy percent less than the NO x  content when the exhaust gas is processed under similar conditions but without the interaction with the SWS vapor stream. 
     
     
         8 . A method of operating a heat generation unit, the method comprising:
 combusting a regeneration (regen) gas stream in a combustion zone of the heat generation unit to produce a combusted regen gas that contains NO x ;   injecting a sour water stripper (SWS) vapor stream that contains ammonia (NH 3 ) as a selective non-catalytic reduction (SNCR) reagent into a post-combustion zone of the heat generation unit; and   allowing the NH 3  of the SWS vapor stream to react with the NO x  of the combusted regen gas in the post-combustion zone of the heat generation unit to produce a processed exhaust gas having a decreased NO x  content.   
     
     
         9 . The method of  claim 8 , comprising receiving the regen gas stream from a fluid catalytic cracking unit, wherein the regen gas stream contains carbon monoxide (CO) and one or more nitrogen-containing compounds. 
     
     
         10 . The method of  claim 9 , wherein the one or more nitrogen-containing compounds are selected from the group consisting of: hydrogen cyanide (HCN), ammonia (NH 3 ), and NO x . 
     
     
         11 . The method of  claim 8 , comprising receiving the SWS vapor stream from a SWS unit, wherein the SWS vapor further contains steam and H 2 S. 
     
     
         12 . The method of  claim 8 , wherein the post-combustion zone of the heat generation unit is disposed downstream of the combustion zone and upstream of a heat recovery zone of the heat generation unit. 
     
     
         13 . The method of  claim 8 , wherein injecting the SWS vapor stream into the post-combustion zone of the heat generation unit comprises:
 injecting the SWS vapor stream in at least two temperature sections of the post-combustion zone, the temperature sections being defined by a range of temperature of the combusted regen gas, and the at least two temperature sections being a first temperature section ranging from about 1600° F. to about 1800° F. and a second temperature section ranging from about 1800° F. to about 2200° F.   
     
     
         14 . The method of  claim 8 , wherein injecting the SWS vapor stream into the post-combustion zone of the heat generation unit comprises:
 injecting the SWS vapor stream in at least three temperature sections of the post-combustion zone, the temperature sections being defined by a range of temperature of the combusted regen gas, and the at least three temperature sections being a first temperature section ranging from about 1600° F. to about 1800° F., a second temperature section ranging from about 1800° F. to about 1900° F., and a third temperature section ranging from about 1900° F. to about 1955° F.   
     
     
         15 . The method of  claim 8 , wherein the processed exhaust gas contains less than 300 parts per million by volume-dry (ppmvd) of NO x , less than 1 part per million by volume-wet (ppmvw) of CO, and about 20 ppmvd or less of NH 3 . 
     
     
         16 . The method of  claim 15 , wherein the processed exhaust gas contains about 100 ppmvd or less of NO x  and about 5 ppmvd or less of NH 3 . 
     
     
         17 . The method of  claim 8 , wherein the decreased NO x  content of the processed exhaust gas is at least 30% less than a NO x  content of the combusted regen gas. 
     
     
         18 . A heat generation system, comprising:
 an exhaust gas conduit for conveying an exhaust gas from a cracking unit to a furnace unit;   the furnace unit containing (i) a combustion zone to receive the exhaust gas and facilitate combustion of the exhaust gas to produce a combusted exhaust gas, (ii) a post-combustion zone equipped with a plurality of inlets to inject a plurality of sour water stripper (SWS) vapor streams from a SWS unit into the combusted exhaust gas from the combustion zone to produce a processed exhaust gas, and (iii) a heat recovery zone to capture heat from the processed exhaust gas; and   a vent conduit for emission of the processed exhaust gas after passing through the heat recovery zone, the processed exhaust gas containing a decreased NO x  content as compared to a NO x  content when the exhaust gas is processed under similar conditions but without an interaction with the plurality of SWS vapor streams.   
     
     
         19 . The heat generation system of  claim 18 , wherein the plurality of inlets includes at least one inlet disposed in each of at least two temperature sections in the post-combustion zone, the temperature sections being defined by a range of temperature of the combusted exhaust gas, and the at least two temperature sections being a first temperature section ranging from about 1600° F. to about 1800° F. and a second temperature section ranging from above 1800° F. to about 2200° F. 
     
     
         20 . The heat generation system of  claim 18 , wherein the plurality of inlets includes at least one inlet disposed in each of at least three temperature sections in the post-combustion zone, the temperature sections being defined by a range of temperature of the combusted exhaust gas, and the at least three temperature sections being a first temperature section ranging from about 1600° F. to about 1800° F., a second temperature section ranging from above 1800° F. to about 1900° F., and a third temperature section ranging from above 1900° F. to about 1955° F. 
     
     
         21 . The heat generation system of  claim 18 , wherein an additional SWS vapor stream from the SWS unit is supplied to the combustion zone and processed along with the exhaust gas to produce the combusted exhaust gas. 
     
     
         22 . The heat generation system of  claim 18 , wherein the NO x  content in the processed exhaust gas is about thirty percent less than the NO x  content when the exhaust gas is processed under similar conditions but without the interaction with the plurality of SWS vapor streams. 
     
     
         23 . The heat generation system of  claim 18 , wherein the NO x  content in the processed exhaust gas is about fifty percent less than the NO x  content when the exhaust gas is processed under similar conditions but without the interaction with the plurality of SWS vapor streams. 
     
     
         24 . The heat generation system of  claim 18 , wherein the NO x  content in the processed exhaust gas is about seventy percent less than the NO x  content when the exhaust gas is processed under similar conditions but without the interaction with the plurality of SWS vapor streams. 
     
     
         25 . A system, comprising:
 a sour water stripper (SWS) unit configured to generate a SWS vapor stream that contains ammonia (NH 3 ) and steam; and   a carbon monoxide (CO) boiler containing a combustion zone and a post-combustion zone, the post-combustion zone having selective non-catalytic reduction (SNCR) nozzles configured to receive and inject a portion of the SWS vapor stream into a combusted exhaust gas from the combustion zone to produce a processed exhaust gas having decreased NO x  content.   
     
     
         26 . The system of  claim 25 , comprising a fluid catalytic cracking unit in fluid communication with the CO boiler and configured to provide a regeneration (regen) gas stream to the combustion zone of the CO boiler, wherein at least the regen gas stream is combusted in the combustion zone to generate the combusted exhaust gas. 
     
     
         27 . The system of  claim 26 , wherein a second portion of the SWS vapor stream is combined with the regen gas stream before being combusted together in the combustion zone to generate the combusted exhaust gas. 
     
     
         28 . The system of  claim 25 , wherein the processed exhaust gas contains less than 300 parts per million by volume-dry (ppmvd) of NO x , less than 1 part per million by volume-wet (ppmvw) of CO, and about 20 ppmvd or less of NH 3 . 
     
     
         29 . The system of  claim 25 , wherein the CO boiler is a wall-fired CO boiler or a tangentially-fired CO boiler. 
     
     
         30 . The system of  claim 25 , wherein a molar ratio of NH 3  to NO x  in the post-combustion zone is between about 1.3 and about 3.8.